Various methods for the after-treatment of exhaust gases exist in order to comply with the increasingly stricter emission limit values that are being set for internal combustion engines, particularly in motor vehicles. Especially the after-treatment of the exhaust gases of diesel engines constitutes a problematic aspect in this context since here, the engine operating conditions where the lambda values are markedly greater than 1 do not permit a simultaneous reductive and oxidative after-treatment. For this reason, in order to reduce the nitrogen oxides (NOx) contained in the exhaust gas, catalytic converters, among other things, are employed for a selective catalytic reduction (SCR) of the nitrogen oxides. However, ammonia has to be added as an additional component for the reaction that takes place in the catalytic converter.
As a rule, the ammonia is injected in the form of a 32.5% aqueous urea solution from a reservoir present in the vehicle. This urea solution is uniformly referred to by the industry as ADBLUE or else as Diesel Exhaust Fluid (DEF) in the United States, and its formulation is described in German standard DIN 70070.
Since the freezing point of this urea solution is at about −11° C., however, the risk exists that the solution might freeze in the vehicle's tank, depending on the season of the year or on the region involved.
For this reason, the state of the art describes the approach of equipping the liquid container with a heating arrangement which prevents the freezing or the jellification of the urea that precedes its complete freezing and which also allows frozen urea solution to thaw at the time of a cold start. Familiar measures comprise the subsequent insertion of a separate heating coil in the container after it has been shaped. Subsequently installed heating elements, however, entail the drawback that the heating performance and the spatial distribution of the heating element in the interior of the container are limited by the size of the opening that is provided in the container wall for the installation of the heating element. Compensating for the insufficient heat-exchanging surface area by setting a higher feed temperature, however, is only possible to a limited extent since the urea solution becomes unusable at elevated temperatures due to thermal degradation. This is why local overheating has to be avoided.
Another approach consists of integrating a heating arrangement directly into the container during its production. British patent application GB 2 136 098 A discloses a heatable tank that is made of plastic by means of rotational molding and that has a heating coil. The heating coil is secured inside the tank by means of connectors that are integrated into the side walls of the water tank. The heating coil installed in the tank exits from the tank at two places where it is held by a clamped connection using the connectors. The connectors are fused into the side wall of the tank during the production process and they have one end which projects towards the outside of the tank and which can be used to connect lines for feeding in or discharging heating media.
European patent specification EP 1 640 577 B1 discloses a heatable plastic tank in which the heating medium inlet and the heating medium outlet of a heating spiral that projects into the tank have plastic sintered around them in the lower area on the plastic wall of the tank. This causes the heating spiral to be immovably affixed in the tank. German utility model DE 20 2006 001 760 U1 discloses a plastic container that serves to hold a urea-water mixture in which piping is integrated or embedded as a heating element directly into the container wall and is connected to the cooling system of an engine. A problem of integrated systems, however, is that holes are needed in the container wall for the electric or liquid-carrying lines in order to operate the heating spiral since these holes entail the risk of leakage.
In order to dispense with lines that pass through the container wall, German patent application DE 10 2010 042 985 A1 puts forward a tank arrangement with a heating arrangement having a heating element in the form of a metal element that is arranged in the interior of the tank and that is designed to provide inductive heating. An electrically actuatable magnetization unit is provided adjacent to the heating element and it is separated from the heating element by the container wall. The solution proposed in this publication is based on the principle of inductive heating of electrically conductive elements. The magnetization unit generates a magnetic alternating field that excites the metal element. The magnetic field creates eddy currents in the metal element and they heat up the metal of the metal element by virtue of the ohmic resistance. In this process, the heat is created directly in the metal element itself and does not have to be transmitted by heat conduction. The metal element arranged on the inner wall of the container transmits the heat directly to the liquid. Generally speaking, this approach can be described as inductive heating of the fluid. The advantage consists especially in that it is possible to dispense with electric lines that pass through the container wall to the heating element that is arranged in the interior.